Relationships Between Rainwater Composition and Synoptic Weather Systems Deduced from Measurement and Analysis of Hong Kong Daily Rainwater Data

Abstract

The composition of Hong Kong rainwater has been studied in the context of seven types of prevailing synoptic weather system. These represent different precipitating airmass origins in terms of continental, maritime, local or mixed sources. Replicate sampling was performed on a daily basis from 1994–1995 for both bulk deposition (n = 60) and simultaneous wet deposition (n = 27) collectors. Reliable analytical data were obtained for the concentrations of the soluble components H+, Na+, Ca2+, K+, Mg2+, Cl-, NO3- and SO42-. The acidity of rainwater was found to be greater when Hong Kong was under the influence of an approaching cyclone (type AC), northeasterly monsoon (type NE) or cold front (type CF) than for prevailing easterly anticyclones (type E), southerly/southwesterly monsoon systems (type S), or cyclonic flows (type C). A common characteristic of occasions of acid rain in Hong Kong was the build-up of local pollutants under conditions of low windspeed, as shown by the significant correlation between rainwater pH and lognormal surface windspeed, but not windvector. The acidity of Hong Kong rainwater was thus largely determined by local sources, although the participation of a continental airmass was evident for some systems. The daily bulk deposition flux of H+ on rainy days varied by more than an order of magnitude from type C (0.2 meq m-2 d-1) to types AC, trough (T) and NE (>2 meq m-2 d-1).